A technology disclosed herein relates to a storage system including a cache memory, and more particularly, to a storage system including two kinds of cache memories, that is, volatile and non-volatile cache memories.
Conventionally, a storage system including hard disk drives has been provided with a cache memory which temporarily stores data read or written by a host computer, thereby increasing speeds of reading and wring data. As a memory device used for the cache memory, conventionally, a dynamic random access memory (DRAM), which is volatile but is much faster than the hard disk drive, has generally been used. However, the DRAM is extremely expensive in terms of cost per bit compared with the hard disk drive, and thus the DRAM of an excessively large capacity cause a problem of an extremely large cost of the system. Moreover, because the DRAM is a volatile memory device, in many cases, the DRAM is provided with a battery for data retention in case of a power down. However, when a DRAM of a large capacity is provided, it is necessary to provide the battery of a large capacity as well, resulting in a problem of a further increase in the system cost.
In order to address such a problem, recently, a non-volatile memory device or, for example, a flash memory, is used as a memory device of the cache memory of a storage system in some cases. The speed of reading data from the non-volatile memory device of this kind is higher compared with the hard disk drive, but the speed is lower than that of the DRAM, and the speed of writing data is even lower than that of the DRAM. To handle this problem, a storage system may be provided with, in addition to non-volatile memory devices, memory devices such as the DRAM which is volatile but has high speed in writing, and may control these volatile memory device so that write data to the non-volatile memory device is temporarily buffered in the volatile memory device, for example, refer to US 2007/0050540. This technology equalizes fine and coarse rates of write to the non-volatile memory device.